D.c. static switch including means to suppress transient spikes between a drive source and the switch element

ABSTRACT

A low voltage drop, D.C. static switch with improved controllability for current limiting is provided wherein the transient spikes from a drive source such as a free running, core-timed oscillator are suppressed by one or more static amplifier stages before the drive signal is supplied to the power switch element. A current limit controller can be isolated from the transient spike source. Such switches have good characteristics for remote power controllers.

United States Patent [1 1 Baker [54] D.C. STATIC SWITCH INCLUDING MEANSTO SUPPRESS TRANSIENT SPIKES BETWEEN A DRIVE SOURCE AND THE SWITCHELEMENT [75] Inventor: Donal E. Baker, Lima, Ohio [73] Assignee:Westinghouse Electric Corporation,

- Pittsburgh, Pa.

[22] Filed: March 15, I971 [21] Appl. No.: 124,310

52 us. or. ..323/9, 321/2, 323/22 T 51 Int. Cl ..cosr 1/58 [58] Field ofSearch ..317/20, 33 R; 321/2, 25;

323/9, 22 T, 38, DIG. 1

[56] References Cited UNITED STATES PATENTS 3,609,512 9/1971 Lewis.,....323/9 CURRENT 1 Jan. 9, 1973 3,377,546 4/1968 Schott ..323/22 T3,513,378 5/1970 Kemper ..323/9 3,373,334 3/1968 Gcisz et al.. ....32l/23,290,583 12/1966 Sinclair ..323/9 Primary Examiner-A. D. PellinenAtt0rneyA. T. Stratton, F. P. Lyle and Gordon H. Telfer [57] ABSTRACT Alow voltage drop, D.C. static switch with improved controllability forcurrent limiting is provided wherein the transient spikes from a drivesource such as a free running, core-timed oscillator are suppressed byone or more static amplifier stages before the drive signal is suppliedto the power switch element. A current limit controller can be isolatedfrom the transient spike source. Such switches have good characteristicsfor remote power controllers.

6 Claims, 4 Drawing Figures PATENTEU JAN 9 I875 3.710.231 SHEET 2 [1F 2El -l I DIRECT VOLTAGE SOURCE s4 v 0| 32 I 33 LIL 35 T as I s} $5 3 4 cm38 V 1 42 1'3; LOAD CURRENT v CURRENT LIMIT i: -i SENSOR CONTROLLER 43 14O LOAD ON/OFF 3 CONTROL INPUT I B L 4. IL

TlME- D.C. STATIC SWITCH INCLUDING MEANS TO SUPPRESS TRANSIENT SPIKESBETWEEN A DRIVE SOURCE AND THE SWITCH ELEMENT BACKGROUND OF THEINVENTION characteristics 1. Field of the Invention The inventionrelates to D.C. static switching circuits for use in electrical powersystems.

2. State of the Art The advantages of static (e.g. semiconductor)switching devices as compared with electromechanical switching deviceshave been previously recognized. One application in which staticswitches are of immediate interest is in aircraft or spacecraft powersystems. Performance qualities including fasttrip-out time during largefaults, ability to limit inrush currents, ability to switch loadsremotely, and generally protect loads and conductors are highlyadvantageous and important in that type of power system. These qualitiesare inherently easier to obtain with static devices than withconventional mechanical thermal circuit breakers.

Some power systems, particularly aircraft power systems, require D.C.power controllers, one for each of the many loads that may be in thesystem, that are capable of limiting the maximum current flow when anyfault is present. A definite limit on fault current means the system canbe built with the smallest size electrically conducting wire that willsafely carry the maximum normal current. Because of the miles ofwirethat may be necessary, the effect of wire size on total system weightand cost is great. Current limiting is also an important factor toreduce system transients. It is clear that static switches can providethe desired current limiting functions yet there have previously beenproblems in doing so without adversely affecting other aspects ofperformance. Besides having a definite limit on current magnitude, it isnecessary that the D.C. power controller provide ripple-free, low noise,controllable current to the load during overload or faulted conditions.Previously available techniques for the use of static switches in D.C.power controllers do not readily meet these requirements withoutsacrificing speed or substantially increasing cost and complexity.

FIG. 1 is a schematic diagram of a circuit that has been previouslyproposed. Here the transistor 10, an NPN power transistor, has itscollector connected to the positive side of a direct voltage source andits emitter connected to the load to be controlled, the other side ofthe load being grounded. Transistor can perform Class Aamplifieroperation up to saturation. The performance of these functionsis determined by the drive current and voltage that is supplied to thebase of transistor10. In the circuit of FIG. 1, a free running,core-timed oscillator, enclosed by dashed line 16, is used that includesa saturable core transformer 12 with a primary center tapped winding 13and two separate secondary center tapped windings l4 and 15.

A constant current regulator 18, which is some means for supplyingconstant current to the center tap of winding 13, is connected betweenthe direct voltage source'and the center tap ofwinding 13. To formthefree running, core-timed oscillator; one outer tap of winding 13 isconnected to ground through the collector-emitter path of transistor 20and also through recenter tap of winding 14 is connected to the emitterof transistor 10.

A current limiting controller 26 is connected across secondary winding15. The separate winding 15 is used to divert part of the base drivecurrent away from winding 14 and the base of transistor 10 as loadcurrent, e.g.

sensed by resistive shunt 27, builds up. In this manner,

the transistor 10 can be driven to a point less than saturation,permitting it to operate as a Class A amplifier, sometimes referred toherein as the current limiting mode of operation in contrast withsaturation which is a full on condition.

I Circuits of the type illustrated in FIG. 1 give acceptable efficiencyand voltage drop for the mode in which transistor 10 is fully on.However, when the current limit controller 26 causes a reduction in basedrive current, the output is not acceptable because of excessive rippleand noise in the current delivered to the load. This results primarilyfrom the fact that practically realizable transformers have somenoticeable leakage inductance causing voltage spikes and current spikeswhich could be avoided only by substantially affecting the speed ofoperation.

FIG. 2 is a set of waveforms for various voltages and currents in thecircuit of FIG. 1 to illustrate the referred to spikes during class Aoperation of transistor 10. V p is the voltage across half the primarywinding 13. V is the voltage across half the secondary winding 14. V isthe voltage between the base and emitter of transistor 10. I is the basedrive current for transistor 10. I is the collector current oftransistor 10 and is the output current supplied by the power controllerto the load. It can be seen that in relative magnitude the spikes in 1are large and this is most undesirable.

It is apparent that modification of the FIG. 1 circuit merely to controlthe constant current regulator 18 directly rather than through winding13 has the same drawbacks. In either case the mode of control of thedrive current to transistor 10 is the same, through transformer 12, andpermits no means of controlling or limiting voltage and current spikeson the secondary side of the transformer.

Another type of D.C. static switch is that disclosed by the presentinventor in copending application Ser. No. 92,348, filed Nov. 24, 1970,now U.S. Pat. No. 3,671,844 issued June 20, 1972 and assigned to thepresent assignee. The apparatus of the copending application is intendedfor full on or full off operation only. If operated in a currentlimiting mode it would have drawbacks similar to those of the circuit ofFIG. 1.

SUMMARY The present invention provides improvement over the prior artexemplified by the circuit of FIG. 1 by developing the current limitingcontrol signals from a current limit controller to a power switch drivecircuit directly rather than through a saturable core transformer. Sucha transformer is, however, retained in preferred circuits in accordancewith this invention for driving the main switching transistor. Thevoltage and current spiking condition still occurs because of the natureof the transformer but now these spikes are not coupled into the powertransistor. The result is an improved power controlling device capableof providing nearly ripple and noise free D.C. current.

DRAWING FIG. 1 is'a circuit schematic of a prior art circuit relevant tothis invention which has been discussed previ- 'ously herein;

FIG. 2 is a set of waveforms of voltages and currents occurring duringoperation of the circuit of FIG. 1;

FIG. 3 is a circuit schematic of an exemplary embodiment of the presentinvention; and

FIG. 4 is a set ofwaveforms of voltages and currents occurring duringoperation of the circuit of FIG. 3.

PREFERRED EMBODIMENTS Referring to FIG. 3, a DC. static switch circuitor power controller in accordance'with this invention is shown. A threeelectrode static switching and amplifying device, transistor Ql, is themain power switching transistor for controlling application of powerfrom a direct voltage source 30 to a load 40. In this example, O1 is anNPN transistor with the collector, base and A emitter identified aselements 31, 32 and 33, respec-' whose outermost taps 38 and 39 areconnected with resistors R5 and R6 and transistors 05 and Q6 in theknown manner for a free running core-timed oscillator, as described inconnection with FIG. 1. The center tap 41 of winding 37 is connected tocircuit point 34 and also is coupled through capacitor C1 to theemitters of Q5 and Q6. Intermediatetaps 42 and 43 of winding 37, locatedsymmetrically about center tap 41, are connected respectively to theanodes of diodes CR1 and CR2 whose cathodes are connected to circuitpoint 44 and to common line 45.

Additional static switching and amplifying devices include PNPtransistor 02 which has its emitter connected'to line 45 anditscollector connected to the base 32 of 01 as well as to line 35through resistor R1. NPN transistor 03 has its collector connected toline 45 through resistor R2 with a direct connection from the collectorof O3 to the base of Q2. The emitter of O3 is connected to line 35. PNPtransistor 04 has its emitter connected I to line 45 and its collectorconnected to the base of Q3 and also through resistor R3 to line 35. Thebase of O4 is connected through, resistor R4 to line 45 and also throughresistor R8 to a current limit controller 46 tobe described. ThedevicesQ2, Q3 and Q4 are connected in cascaded amplification stages limitcontroller means 46. A resistor R7is connected to the emitters oftransistors Q5 and Q6 and to the collector of an input controltransistor Q7 whose grounded.

In operation, the free running core-timed oscillator emitter is 36supplies current through CR1 and CR2 to amplifier circuit 47 rather thandirectly to the base 32 of Q1. The. ratio of turns N to N, of winding37, where N is the number of turns between taps 38 and 41 and N, is thenumber of turns between taps 41 and 42 and the value of R7 are selectedto provide a desired current level on line 45 as will be described.

The switch under a given set of requirements must be capable of passingup to 1.5 (although it could be some other artibrary value)'times normalcurrent before current limiting occurs. When these conditions are met,and transistor O4 is biased on, the entire power switch consisting ofQ1, Q2, Q3 and Q4 will be on. The forward voltage drop across 01 will beequal to or less than that attained with the prior art circuit of FIG. 1for all normal load conditions. As load current increases from zero,increased base drive to Q1 is necessary.

Under overload conditions, base current in excess of the oscillatorscapacity is provided from the supply 30. The switch has a higher voltagedrop for overload conditions, as is also the case with the prior artcircuit, and this is acceptable. With circuits of the prior art,overloads require higher base drive current to the transistorcorresponding to Q1 that adversely affects efficiency.

During the current limiting mode of operation, the

small as desired by adding amplification stages between winding (towhich taps 41, 42 and 43 would be con-- nected) can be used instead ifdesired to permit-alines 35 and 45. In the illustrated example, a totalof three such stages, Q1, Q2, and Q3 are used. If 10 amperes of loadcurrent are to be controlled, the magnitude of I should be 1 milliampereor less. In this range, it is feasible to use linear integrated circuitsfor the current limiting controller which wasnot permissible with theprior art circuit.

Also, during the current limiting mode of operation, the oscillatoroutput current is not directly connected to power transistor Q1. When Q1is not in full saturation, and is in the current limiting mode, O2 isalso not in saturation. When Q2 is in the linear portion of itsoperating characteristic, it appears essentially as a current source toits load, Q1. Consequently, positive voltage spikes from the output ofT1 do not cause current spikes into the base of Q1.

In the example of FIG. 3, an autotransformer configuration is used forT1, that is with a single, multitapped winding 37. However, an isolatedsecondary greater choice in the manner in which the load current issensed forthe current limit controller. In the Current. limit controller46 can take variousforms in the practice of this invention. Its purposeis to control the magnitude of current I which has an inverse effect onthe magnitude of load current I Controller 46 could be a manuallycontrolled resistor or a transistor connected to vary l as needed.Preferably controller 46 controls the voltage across R8 and, hence, thecurrent lg by a load current detector. The detector may be anoperational amplifier with a resistive shunt at some point on the loadcircuit between points 34 and 35 sensing I The operational amplifier ispreferably in a configuration to reduce I if I is to exceed apredetermined level in a manner not dependent on load impedance. Furtherdescription of a preferred current limit controller 46, useful in thecircuit of FIG. 3 as further described in the following detailedexample, may be found by referring to above mentioned copendingapplication Ser. No. 124,232.

FIG. 4 shows the waveforms for various voltages and currents of .thecircuit of FIG. 3 during the current limiting mode of operation. Thesimilarities and differences with the corresponding functions of theprior art circuit (as shown in FIG. 2) can be readily noted; I does nothave transient spikes.

A circuit as illustrated in FIG. 3 has been constructed and successfullyoperated. By way of further example, the following components aresuitable.

Transistor Q l Westinghouse Type 125, 2N2l l7 Transistor Q2 2N3790Transistor Q3 2N344l Transistor Q4 2N4829 Transistor Q5 2N2243Transistor Q6 2N2243 Transformer T] 1 1:1 turns ration, core MagneticsInc. 50056-ID Diode CRI IN4942 Diode CR2 IN4942 Resistor R1 47 ohmsResistor R2 56 ohms Capacitor CI 5.6 microfarads Resistor R3 1,000 ohmsResistor R4 2,000 ohms Resistor R5 5,100 ohms. Resistor R6 5,100 ohmsResistor R7 80 ohms Resistor R8 10,000 ohms Load 40 2.8 ohms nominalDirect Voltage Source 30 VDC nominal Typical operating characteristicsof a circuit as described are:

Voltage drop: 0.32 Volts Electrical Efficiency: 97 to 98 percent at fullload.

Speed: less than 20 microseconds response time.

Current limiting capability: l5 amperes.

Control power level: 90 milliwatts (current limit controller) RippleContent: less than one percent Noise Content: less than one percent 7High fault current at normal efficiencylimited by Q1, could be as highas 30 amperes for Q specified.

These operating characteristics are found to compare favorably with thecorresponding characteristics of prior art circuits.

In the circuit of FIG. 3, sufficient current regulation is provided byresistor R7. A constant current regulator 18, as indicated in FIG. 1,which normally requires an additional active element e.g. transistor),is unnecessary.

Iclaim:

1. Apparatus to control the application of D.C. power from a supplycircuit point to a load circuit point, comprising:

a first static switching and amplifying device having first, second andthird electrodes of which said first electrode is connected to thesupply circuit point and said third electrode is connected to the loadcircuit point, said second electrode being a control electrode, atransformer having a saturable core and at least one winding thereonhaving a plurality of turns, a first number of turns disposedsymmetrically about a center tap on said at least one winding connectedat symmetrical taps respectively through like poled diode rectifiers toa common line, said center tap being connected to said supply circuitpoint; at least one additional static switching and amplifying device inat least one amplification stage having an input from said common line,an output to said second electrode of said first static switching andamplifying device, said second electrode being isolated from said commonline by said at least one amplification stage, and said at least oneamplification stage also having a control point connected to currentlimit control means for permitting variable control of the amount ofsignal applied to said second electrode of said first static switchingand amplifying device without any appreciable spikes in the currentapplied to said load circuit point.

2. The subject matter of claim 1 wherein:

said static switching and amplifying devices are transistors, saidsecond electrode of said first device being the base electrode thereof;and further comprising a free running, core-timed oscillator includingsaid transformer for developing signals to said turns connected to saidlike poled diode rectifiers.

3. The subject matter of claim 2 wherein said current limit controlmeans is electrically isolated from said at least one winding of saidtransformer and is not influenced by transient spikes developed in saidtransformer.

4. In an electrical power system, the combination comprising:

a power transistor with a collector-emitter path connected between asource of direct voltage and a load, a transistor amplifier circuitconnected to the base of said power transistor; means including a freerunning, core-timed oscillator for developing signals applied to theinput of said transistor amplifier circuit; current limit control meansalso connected to the input of said amplifier to permit modification ofcurrent supplied to the base of said power transistor; said freerunning, core-timed oscillator including a transformer exhibitingcharacteristic leakage inductance evidenced as voltage spikes on itswindings upon going in and out of saturation; said current limit controlmeans and the base of said power transistor being effectively isolatedfrom said voltage spikes by said amplifier circuit.

5. Apparatus to control the application of D.C.

power from a supply circuit point to a load circuit point, comprising:

static switching means having first and second electrodes between supplyand load circuit points and a control electrode; means to develop a DC.drive signal characterized by appreciable transient current spikes; andmeans to suppresssaid transient current spikes connected intermediatesaid D.C. drive signal means and said control electrode,said means tosuppress said transient current spikes comprising at least one staticamplifier stage and a current limit controlmeans to control the magnitude of drive current to said at least one static

1. Apparatus to control the application of D.C. power from a supplycircuit point to a load circuit point, comprising: a first staticswitching and amplifying device having first, second and thirdelectrodes of which said first electrode is connected to the supplycircuit point and said third electrode is connected to the load circuitpoint, said second electrode being a control electrode, a transformerhaving a saturable core and at least one winding thereon having aplurality of turns, a first number of turns disposed symmetrically abouta center tap on said at least one winding connected at symmetrical tapsrespectively through like poled diode rectifiers to a common line, saidcenter tap being connected to said supply circuit point; at least oneadditional static switching and amplifying device in at least oneamplification stage having an input from said common line, an output tosaid second electrode of said first static switching and amplifyingdevice, said second electrode being isolated from said common line bysaid at least one amplification stage, and said at least oneamplification stage also having a control point connected to currentlimit control means for permitting variable control of the amount ofsignal applied to said second electrode of said first static switchingand amplifying device without any appreciable spikes in the currentapplied to said load circuit point.
 2. The subject matter of claim 1wherein: said static switching and amplifying devices are transistors,said second electrode of said first device being the base electrodethereof; and further comprising a free running, core-timed oscillatorincluding said transformer for developing signals to said turnsconnected to said like poled diode rectifiers.
 3. The subject matter ofclaim 2 wherein said current limit control means is electricallyisolated from said at least one winding of said transformer and is notinfluenced by transient spikes developed in said transformer.
 4. In anelectrical power system, the combination comprising: a power transistorwith a collectOr-emitter path connected between a source of directvoltage and a load, a transistor amplifier circuit connected to the baseof said power transistor; means including a free running, core-timedoscillator for developing signals applied to the input of saidtransistor amplifier circuit; current limit control means also connectedto the input of said amplifier to permit modification of currentsupplied to the base of said power transistor; said free running,core-timed oscillator including a transformer exhibiting characteristicleakage inductance evidenced as voltage spikes on its windings upongoing in and out of saturation; said current limit control means and thebase of said power transistor being effectively isolated from saidvoltage spikes by said amplifier circuit.
 5. Apparatus to control theapplication of D.C. power from a supply circuit point to a load circuitpoint, comprising: static switching means having first and secondelectrodes between supply and load circuit points and a controlelectrode; means to develop a D.C. drive signal characterized byappreciable transient current spikes; and means to suppress saidtransient current spikes connected intermediate said D.C. drive signalmeans and said control electrode,said means to suppress said transientcurrent spikes comprising at least one static amplifier stage andcurrent limit control means to control the magnitude of drive current tosaid at least one static amplifier stage in accordance with variationsin magnitude of current between said supply and load circuit points. 6.The subject matter of claim 5 wherein: said means to develop a D.C.drive signal characterized by appreciable transient current spikescomprises a free-running core time oscillator; and said static switchingmeans and said at least one static amplifier each comprise a transistor.